feat: use stable preview URLs and skip draft PRs

[fern-support]

feat: use stable preview URLs and skip draft PRs

Summary

Part of a cross-repo feature to make docs preview URLs stable across PR commits (see fern#10575).

Two changes to the preview-docs workflow:

1. Stable preview URLs: Passes --preview-id with the PR number so every commit in the same PR reuses the same preview domain instead of generating a new one.
2. Skip draft PRs: Adds if: github.event.pull_request.draft == false to avoid wasting preview generations on draft PRs.

⚠️ Depends on companion PRs being merged and released first:

• fern-platform (server): accepts previewId and derives a deterministic domain
• fern CLI: adds --preview-id flag support

Review & Testing Checklist for Human

• Deployment order: This workflow will fail if merged before the Fern CLI release that supports --preview-id. Confirm the CLI change is released before merging.
• Verify the draft PR skip condition (github.event.pull_request.draft == false) behaves correctly — note that PRs marked as "ready for review" after being draft will re-trigger the workflow.
• End-to-end test: open a PR against a repo using this workflow template, push two commits, and confirm both produce the same preview URL in the PR comment.

Notes

• Devin Session
• Requested by: @fern-support

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